The low-power radio signal from car keyfobs can be drowned out by the stronger transmissions of the masts, which use a similar frequency.

The RAC and the AA say that they deal with thousands of stranded motorists whose cars have been locked or disabled because of phone masts.

Cars are at risk from every type of phone mast but there are particular difficulties with the new high-powered police digital radio system, Tetra (terrestrial trunk radio), introduced nationally as part of a project known as Airwave.

A briefing document for ministers at the Department of Trade and Industry (DTI) says: “This problem has existed for some years, although with the roll-out of Airwave, it is not surprising we are now getting more reports of this. These stronger sources of radio transmission can sometimes block or overwhelm the signal from a radio-activated key.”

Range Rovers and cars made by Vauxhall, Chrysler, Jeep and Saab are believed to be especially at risk. Older models are vulnerable because they cannot differentiate between a keyfob and a mobile-phone signal. Motorists are advised to change the fob battery regu- larly and park away from masts and aerials to reduce risk.

The AA was called out to 114,000 key-related problems last year and the RAC dealt with 70,000 similar security problems, many believed to have been caused by mobile- phone masts.

In most cases the immobiliser can be deactivated by pushing the car 100 yards further away from the mast, but some drivers have to pay a garage about £50 to have the system reset. In the worst cases immobilisers have to be replaced at a cost of up to £600.

In one case, in Manchester, engineers found that the Tetra mast at the Trafford Centre operates at about 400MhZ, while key fobs run at 433MhZ, causing immobilisers to become effectively immune to a sudden burst at a similar frequency.

Vehicle manufactures have begun to introduce better- quality shielding for cars to prevent the interference. The problem has been exposed after briefing papers and letters were released by the DTI after a Freedom of Information Act request. The documents detail an exchange between Norman Lamb, the MP for Norfolk North, acting on behalf of a constituent who was concerned that the Tetra mast at North Walsham police station was to blame for her Land Rover failing to start. In his reply Stephen Timms, the Energy Minister, said that there was a history of car alarms being affected by radio interference.

He said: “If you are using kit that relies on or uses radio frequency it is your responsibility as owner to ensure it cannot be affected by outside radio frequency sources.”

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